Polycarbonate resins have excellent clarity and heat resistance, and a low water absorption, making them ideally suited as optical-grade materials. In optical products such as optical disks, and especially in compact disks, after molding, because aluminum is vapor-deposited onto one side, bonding of the disk substrate with the aluminum is important. Polycarbonates are generally prepared by a phosgene process. However, polycarbonates obtained by means of this process have an inadequate adhesion with aluminum.
Continuous manufacturability is required in the production of optical products such as compact disks and laser disks. Hence, excellent mold releasability is desired in the polycarbonate resins employed in these products. In addition, because the molding temperature for these products is at least 300.degree. C., the polycarbonate resin must have a high heat stability. However, polycarbonates obtained by the phosgene process have a poor mold releasability, as a result of which the continuous manufacturability of molded products based on such polycarbonate is low, which represents a drawback. Polycarbonate compositions are known in which the mold releasability has been enhanced by the formulation of various parting agents, but these polycarbonate compositions present problems in terms of their heat stability.
Also, prior-art polycarbonate resins discolor during molding. Optical-grade materials should preferably be clear and have a small optical distortion. Methods used for obtaining molded product having a small optical distortion consist of improving the resin flow either by using a high resin temperature during molding or by employing a resin having a low average molecular weight; and thereby avoiding tile formation of optical inhomogeneities on account of flow by the molten resin. However, when the resin temperature during molding is high, discoloration due to decomposition of the resin tends to arise. Moreover, when a polycarbonate resin having a low average molecular weight is used, a marked decrease in the molecular weight occurs during molding and problems such as discoloration arise, resulting in a major decline in the mechanical properties.
Hence, the object of this invention is to provide optical-grade polycarbonate resin compositions in which the above drawbacks have been resolved, which compositions have an excellent adhesion with aluminum, a good heat stability, a good mold-releasability and an excellent manufacturability, and which experience little discoloration during molding.